Why would they buy an XB1 so they can play with their friends on PS+?

So they can play with friends who own an XBox1

That's got nothing to do with what we were discussing. As soon as I see a goalpost move I'm out.

We're discussing the ability of XBox 1 and PS4 owners being able to play the same online games together aren't we?

And removes the reason for someone on LIVE to pay for PS+

You are suggesting Sony should throw the reason for people to buy a PS4 and PS+ away.

Not necessarily: a PS4 would still be a viable gaming machine except now owners would have more people they could play with. Their friend with an XBox 1 would be able to play with them.

There are over 40 million PS4s out there and there's never a shortage of players to play against. That number increases every year because in order to play with your friends you have to buy a PS4 and pay for PS+. The only reason MS made that statement (for the first time ever) is because they're losing revenue to Sony because of PS+. You cannot expect any company to just throw away profits because YOU have a different console. That's just silly.

I wouldn't expect them to.

Why wouldn't you expect them to?

As you said, who would throw away potential profits?